The semiconductors industry continues increased interest in electron beam lithography as an alternative to ultraviolet lithography in the manufacture of semiconductor devices. A major advantage of electron beam lithography is in reducing the size of electronic components and circuits. The electron beam technique also offers the advantages of very high resolution, direct writing on the substrate, and lower defect incidences. Direct writing also eliminates alignment or mask substrate contact difficulties. Attainment of such resolution, e.g. greater than 0.5 .mu.m, without the sacrifice of high sensitivity demands the use of dry development processes such as ion milling, sputter etching, plasma etching, and reactive ion etching. This is especially important in the case of negative resists, where swelling in the presence of wet developing solvents may degrade resolution.
The interest in E-Beam technology has also inspired considerable effort in the development of electron sensitive resists of both the positive and negative type. These resists are polymeric solutions that are spin-coated onto the workpiece, dried by baking and exposed to the electron beam in the desired pattern. Resist development is accomplished by soaking in a solvent or mixture of solvents which dissolves either the irradiated portion (positive resist) or the non-irradiated portion (negative resist). The present invention is concerned with negative resists.
Negative resists are significantly more compatible with dry etching processes than are positive resists. Negative resists themselves vary in their dry etch resistance. In addition, structural characteristics such as strong backbone bonds, aromatic rings, polar functional groups and the presence of metals all decrease the rate at which crosslinked polymers are removed from a surface, e.g., by oxygen plasmas. The presence of aromatics also improves resistance to CF.sub.4 and CF.sub.4 /O.sub.2 plasmas. Commercially-available diazo-type resists contain a cresol-formaldehyde resin (novolac) which displays excellent dry etch resistance. Relatively long exposures, .about.2.times.10.sup.-5 coul/cm.sup.2, however, are required for commercially-available resists such as Shipley AZ-1350J, AZ-1370, and AZ-2400, in order to obtain vertical line profiles (i.e., 90% of original unexposed resist thickness remaining) of 1-.mu.m wide lines in 1-.mu.m thick resist. Although this sensitivity is far superior to that of the commonly-used positive resist PMMA, polymethyl methacrylate, (.about.8.times.10.sup.-5 coul/cm.sup.2 required), it is inferior to that of other commercially available negative resists such as COP, OEBR-100, SEL-N and KMER. Due to their superior dry etch resistance, it is highly desirable to increase the sensitivity of the diazo-type resists to the 20-keV electrons commonly used.
The incorporation of metals such as Cs, Tl, Ca, Sr, Ba, and Pb, I Haller, R. Feder, M. Katzakis and E. Spiller, J. Electrochem. Soc. 126(1), 154 (1979) and D. J. Webb and M. Hatzakis, J. Vac. Sci. Technol. 16(6), 2008 (1979) into positive resists such as copolymers of methyl methacrylate and methacrylic acid, has previously been found to increase the sensitivity of the polymer to electrons and to x-rays, raise the flow temperature and improve overall chemical etch resistance. The higher the atomic number of the element incorporated, the greater the degree of improvement. The metal ions may serve as inelastic scattering centers, L. F. Thompson, E. D. Feit, M. J. Bowden, P. V. Lenzo and E. G. Spencer, J. Electrochem. Soc. 121, 1500 (1974), reducing the percentage of the electron beam passing unchanged through the organic resist layer. The doping of PMMA with small quantities of MMA--Me.sub.4 Sn copolymer (up to 2%, volume basis)--has been found to increase the self-development sensitivity eightfold and the contrast by 40%, L. F. Thompson and M. J. Bowden, J. Electrochem. Soc. 120, 1722 (1973) and M. Yamada and S. Hattori, Jpn. J. Appl. Phys. 20(10), 1969 (1981).
As pointed out above, it is known to incorporate metals and metal ions into photoresist compositions. In addition to the literature articles mentioned, U.S. Pat. No. 4,307,178 discloses phenol-formaldehyde Novolak resin/diazo ketone resist layers which have been treated with a magnesium salt to produce a negative pattern. Further, U.S. Pat. No. 3,954,468 discloses a radiation process for producing colored photopolymer systems and indicates in Table 7 various metallo-organic additives which are known to improve electron beam and X-ray response. Additives in this group include triphenylleadbromide, triphenylbismuth, and phenyl mercuric iodide. U.S. Pat. No. 3,594,170 relates to additives for negative photoresists to increase the sensitivity wherein the additives are organometallic compounds, such as dibutyltin maleate. Similar disclosures may be found, for example, in U.S. Pat. Nos. 4,156,745, 3,656,951, 3,374,094, 3,977,874, 4,086,091, and 4,347,304.
In all of these prior art systems, however, the metal incorporated into the photoresist composition is either reacted with the finished photoresist or merely added as an additive material. The present invention provides an improved photoresist composition based on the discovery that such improved compositions are obtained if the polymer from which the photoresist is to be manufactured is prereacted with an organometal compound.